HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesMSExchangeISVirusScan
Set ProactiveScanning = 1 (proactive scanning occurs when an item is delivered to a folder, but a request has not been made by a client)

Set ReloadNow = 1 (By default this value is set to zero, by setting this value to "1", Exchange will reload the configuration settings from the registry and reload the Vendor's DLL. Once completed this value is reset to "0" by Exchange.)